This invention relates generally to film bulk acoustic resonators.
Front-end filters that include film bulk acoustic resonators (FBAR) are advantageous compared to other filter technologies such as surface acoustic wave (SAW) devices and ceramic filters, particularly, at relatively high frequencies. For example, SAW filters may begin to have excessive insertion losses above 2.4 gigahertz (GHz). Ceramic filters are much larger in size and become increasingly difficult to fabricate as frequency increases.
A conventional FBAR filter includes two sets of FBARs in order to achieve the desired filter response. The series FBARs have the same first frequency and shunt FBARs have the same second frequency. The frequency of each FBAR is primarily determined by the thickness of its piezoelectric film. The frequencies of the series and shunt FBARs need to be precisely controlled in order to have the desired filter response, such as the desired central frequency and pass bandwidth.
As an example, a 2 GHz FBAR may have a piezoelectric film thickness of approximately 1.8 micrometers. If a one percent thickness non-uniformity occurs, the frequency of the FBAR may shift 20 megahertz, which may not be acceptable if a 60 megahertz band pass is required.
To overcome the problem of frequency shifts arising from film thickness errors, post-process trimming may be used to correct the frequency. Post-process trimming techniques include using the mass loading effect and etching the top metal layer or depositing more metal. Alternatively, post-trimming techniques may involve the use of a heating element. All of these techniques are die-level processes and, as a result, have relatively low throughput. In addition, in-situ measurement is required during the post-process trimming steps. Therefore, the cost may be relatively high. As a result, die-level post-trimming techniques are not suitable for high volume manufacturing.
Thus, there is a need for a way to improve the frequency uniformity of film bulk acoustic resonators which is more suitable for high volume manufacturing.